1. Field of the Invention
The present invention relates to the generation of short rectangular high voltage pulses of electrical energy, and more particularly, to a novel modulator having a number of pulse forming networks connected so that they may be charged in parallel and discharged in series to provide high voltage pulses with fast rise and fall times.
2. Description of Related Art
Many types of electrical apparatus, such as microwave tubes used in radar transmitters and particle accelerators, require short rectangular high voltage pulses of electrical energy for their operation. These pulses may be on the order of tens or hundreds of kilovolts with a pulse width in the nanosecond or microsecond range. The repetition frequency of the pulses may range from one hundred to several thousand Hertz.
The predominant technology for generating these pulses makes use of a pulse forming network which functions as an artificial transmission line. The network is charged to a desired voltage level and then is coupled to an external load by a switch. Since neither the network or the switch can withstand the high voltage level required by the load, a transformer raises the network voltage to the required level. The switch is activated by external timing signals. This discharges the network into the primary windings of the transformer which provides the stepped up voltage pulse to the load. After each pulse has been transmitted, the charging of the network is repeated.
The switch may be a solid state device, or a gas filled tube such as a thyratron. A thyratron is filled with hydrogen gas and is capable of withstanding about 50 to 100 kilovolts between its cathode and anode. The thyratron has a grid disposed between the cathode and anode, and thyratrons of higher voltage ratings may have a plurality of such grids. Ionization of the hydrogen gas is initiated by a triggering voltage applied to the grid nearest the cathode, which overcomes the negative bias of the grid with respect to the cathode and initiates an arc discharge of current from the cathode to the anode. Thyratrons are generally preferred over solid state devices, since they can withstand a much higher voltage in the "off" state and conduct higher current in the "on" state.
However, a drawback with the prior art pulse generators is that the transformer tends to degrade the quality of the pulse at high voltage levels and short pulse widths. When the transformer step-up ratio is high, the leakage inductance of the transformer is also high, which tends to increase the rise and fall times of short pulses. Thus, it is increasingly difficult to provide rectangular high voltage pulses having very fast rise and fall times.
Accordingly, it would be desirable to provide a modulator which overcomes these difficulties of the prior art. It would be further desirable to provide a modulator which does not use a transformer to increase the voltage level above that which the network can withstand.